1. Field of the Invention
The present invention relates to an image processing apparatus capable of reproducing a smooth halftone image while efficiently using the storage area.
2. Description of the Related Art
As is well known, in storing multi-value image data, it is binarized to reduce its data amount to thereby efficiently use the storage area. However, the binarization deteriorates the image quality. This is remarkable particularly in a halftone image. Conventionally, to minimize the image quality deterioration of a halftone image in reducing the data amount, various types of quasi-halftone processing such as the dither method and the error diffusion method have been used.
For example, Japanese Unexamined Patent Publication No. Sho. 64-18368 discloses an improved version of the density pattern method. Japanese Unexamined Patent Publication No. Sho. 63-102469 a technique in which image data is stored with its resolution reduced to 1/N, and an image is reproduced after the resolution is increased by linear interpolation by a factor of M.
However, in the technique of the publication Sho. 64-18368, when the amount of image data is reduced to a level comparable with that of two-value image data, the image quality of a halftone image is deteriorated. In the technique of the publication Sho. 63-102469, the definition of a resultant image is decreased, so that the quality of an image constituted of characters or a line drawing is deteriorated.
Which of the resolution and the number of gradation levels is more important than the other to keep a satisfactory image quality depends on the nature of an image to be processed. For example, because of its nature, it is desirable that an image containing characters or a line drawing be reproduced finely. Therefore, importance is attached to the resolution, while the number of gradation levels is considered less important. On the other hand, for an image constituted of a photograph or a picture, it is desirable, because of its nature, to reproduce a smooth halftone image. Therefore, the number of gradation levels is considered more important than the resolution.
A technique for accommodating such a difference in the nature of images is disclosed in Japanese Unexamined Patent Publication No. Sho. 63-59674. In this technique, an input image is subjected to area discrimination. Area information as a result of the area discrimination, binarized image information, and resolution-converted multi-value image information are stored, and an image is outputted while switching is made between the two-value image information and the multi-value image information in accordance with the area information.
However, the technique of the publication Sho. 63-59674 is deficient in that a large-capacity storage device is needed to secure separate storage areas for the two-value image information and the multi-value image information.